Turbomolecular vacuum pump



1965 H. 1.. GARNIER ETAL 3,163,978

TURBOMOLECULAR VACUUM PUMP Filed July 30, 1962 V 2 Sheets-Sheet 1 FIG.!

INVENTORS Jihad, Kan} b llu X RM "r I 2W5, .ammum 00M. 0-44., M+

ATTORNEYS 1965 H. GARNIER ETAL 3,163,973

TURBOMOLECULAR VACUUM PUMP Filed July 30, 1962 2 Sheets-Sheet 2- FIG. 2

Y A INVENTORS x z E new Mum/0W R W 1 9-, z, MM

mm, M M! w ll-Q ATTORNEYS United States Patent 15 Claims. (c1. z30 11s The present in ention relates to tur-bomolecular vacuum pumps based on the principle of a compressor required to operate in a very low pressure range corresponding to the molecular system, the delivery of the gas being .obtained by an assembly of successive discs, alternately fixed and rotating, in which there are formed apertures such that the probability of passage of theirnolecules is greater in one direction than in the other. A vacuum pump of this type has been described in US. patent application 189 ,O21 filed by the applicants on May 15, 1961, now Patent No. 3,138,318 and entitled Turbomolecular Vacuum Pump.

An essential characteristic of the present invention resides in the fact that the pump is of single flow type, the suction on the high vacuum being effected through an orifice of large conductance located on one side of the assembly of successive discs, while the delivery towards the preliminary vacuum is effected on the opposite side, the. molecules of rarefied gas thereby having a mean (path substantially without any bend and generally in the direction of the axis of rotation, the rotor of the pump being supported by means excluding any lubricated hearing on the high vacuum side.

According to one embodiment of the invention, the

rotor is mounted on a main bearing disposed close to its centre of gravity and having great bending strength, a secondary bearing associated with the main hearing being placed at a certain distance from the centre of gravity and requiring a lesser bending strength. The mechanical transmission to the frame of the pump of the stresses or forces originating from the rotor through the bearings is etfected entirely on one and the same side or end of the pump and more precisely on the side opposite to the high vacuum, so that it can be said that the rotor is in effect mounted so as to be overhung in the direction of the high vacuum. w

The turbomolecular pump of the present invention has other important characteristics, the most interesting of which appear to bethe following:

The main-bearing positioned close to the centre of gravity of the rotor' is connected to the assembly of rotating discsthrough the medium of a sleeve integral with aplate to which the assembly is fastened and which is located in a transverse plane off-set with respect to that of said bearing; i h

The bearings have their rotating races on the inside and their fixed races on the outside and are interposed between a central driving shaft and a drumsurrounding the latter and fast with the frame of the pump, the driving shaft projecting at the end of the drum and being connected at this level to the rotor through the medium of a sleeve in turn surrounding the drum;

The lubrication circuit of the pump, and more generally the space in which the particles of lubricant are like ,ly to circulate, are isolated in fluid-tight manner from the region of high vacuum-the pump has a vertical axis with the suction orifice at the top, the lubricant accumulating by gravity towards the bottom of the pump in a 3,158,978 Patented Feb. 9, 1965 central region remote from the upstream peripheral region adjacent the high vacuum;

A cold screen is interposed around the space containing the lubricant for the purpose of ensuring condensation of oil vapour which may form and preventing the vapour ascending as far as the level of the high vacuum;

The heating of the pump in order to promote the emission and elimination of the gaseous particles adsorbed by the walls is effected in the presence of the above-mentioned cold screen and when the pump is rotating, for example at half speed, by heating externally, the suction flange and the upstream stages of the pump being advantageously heated in a preferential manner.

Other characteristics of the present invention will be clear from the following description given by way of example with reference to the accompanying drawings, in which:

FIGURE 1 is an axial half section of a turbomolecular vacuum pump in accordance with the invention having a horizontal axis of rotation, and

FIGURE 2 is a similar view of a modified construction with a vertical axis of rotation.

The turbomolecular vacuum pumps illustrated in the drawings each comprise a series of rotating discs 1 between which are interposed fixed discs 2 fixed to the casing 5 of the pump. Through the discs 1 and 2 suitable apertures for the passage of the rarefied gas molecules are formed over opposite annular zones 3 and 4. The arrangement and form of these apertures are not shown in the drawings for reasons of simplification and also because they do not appear to be essential for a good understanding of the present invention; in this connection it would be possible to use with advantage the arrangements forming the subject-matter of our copending U.S. patent application Serial No. 213,416, now abandoned, filed this day by the applicants Improvements in Turbomolecular Vacuum Pumps.

The rotating discs 1 each have an annular portion 6 of extra thickness with plane parallel faces; the thickness of this portion 6. is in fact equal to the pitch or axial distance of recurrence of the discs 1 or 2, so that the rotating discs 1 are stacked without any clearance between the thickened portions 6, as shown clearly in the drawings. In this way, scaling is obtained between a peripheral space A in which the molecules of the rarefied gas circulate and an inner space B in the hub region of the rotating discs 1. The hubs 11 of the discs 1 are likewise of extra thickness, but are arranged with a slight clearance between them so as to avoid the creation of a'large number of small sealed-off chambers between the discs 1.

The discs 1 are assembled by meansfof tie rods 7 parallel to the axis of rotation X-Y and distributed around a pitch circle centered on that axis, so as to preserve symmetry and equilibrium of the assembly. The tie rods 7 extend through the extra thickness portions 6 of the discs 1.

In the embodiment shown in FIGURE 1, the rods 7 also extend through a plate 9 located in front of the adjacent end disc 1 and a plate It) occupying an intermediate position towards the rear of the stack or set of discs 1. The plates 9 and 10 are of extra thickness adjacent their peripheries at' 9a and 10a.

The plate 10, which is of annular form and formed with holes 12 providing communication between the space B and a space C, is also formed integrally with a coaxial sleeve 13 on which there is fixed, by means of screws 14, a solid circular end cover 15 adjacent the plate 9. The sleeve 13 is slidably engaged at the front end 13a over a circular rib 16 on the plate 9, while a slight clearance is left between the inner pen'phery of the rotating discs 1 and the outer surface of the sleeve through the medium of ball bearings 18 and 1%;

' completely isolated from the high vacuum.

oy the end cover 15, the plate 9 and the end 13a of the sleeve 13. The solid circular plate 9 isolates an up "stream space P, which communicates with the chamber to be evacuated and is therefore under a high vacuum,

from the inner downstream spaces B and E which are in turn isolated from the space D inside the sleeve.

The sleeve 13 is fitted over a hollow shaft 17 which is of large diameter over the major part of its length but terminates in a journal 17a of smaller diameter. The shaft 17 is fixed relatively to the casing of the pump and its connection with the rotary sleeve 13 is effected the outer race of the former is housed in the cover- 15 and the outer race of the latter in an annular part 1% fast with the sleeve 13, while the inner races are fixed to the journal 17a of the fixed shaft 17 and linked by means of a spacing sleeve 24.

A plug 20 closes the free end of the fixed shaft 17, which is slightly set in with respect to the rotary cover 15, while small ducts 21, '21 provide communication between the internal space G of the hollow shaft 17 and the ball-race bearings 13 and 19. This arrangement is designed to permit lubrication of the bearings by oil from the space G, return of the oil being effected through the space D inside the sleeve 13 and through ducts 22 formed in the hollow shaft 17 within a collecting bush 23 fixed to the latter.

It is to be observed that, as a result of the sealing between the space D containing oil particles and the inner spaces B and E achieved by the cover 15 and the sleeve 13 and the sealing between these latter spaces and the upstream space F achieved by the plate 9, the ballrace bearings 18 and 19 and the lubricating system are In fact, it is essential that no trace of oil should reach the chamber to be evacuated.

The driving of the rotary assembly constituted by the discs 1 and the support 9, 10, 13 to which they are conneoted by the tie rods 7 is effected by means of an electric motor incorporated in the pump. This motor, which is of the axial plane gap type comprises, on the one hand, a stator 25 in the form of a disc fixed to the hollow shaft 17 and carrying the motor windings which may be constituted by a fiat coil with lamellar conductors and the connections to the external supply circuit and, on the other hand, a rotor which may be constituted by toroidal magnets 26, 26 made, for example, of ferrite. The magnets are fixed to a cylindrical frame or casing 27 having a flange 28 against which the heads 29 of the tie rods 7 engage, while nuts 30 screwed on to the threaded ends 8 of the rods 7 bear against the terminal plate 9.

It is appropriate to observe the special arrangement of the whole of the rotary part of the pump which is overhung on a fixed shaft. This arrangement enables any risk of vibration to be eliminated under normal running conditions and also during acceleration. This result takes account of the fact that the critical speed of the assembly must be higher than the speed of rotation, in spite of the use of ball bearings of small diameter which is necessitated by the high speeds of rotation.

The solution of this problem has been facilitated by the great bending strength obtained by arranging the bearing 19 close to the centre of gravity of the rotating part. Immediately after this hearing, on the side where it is fitted in, the fixed shaft 17 exhibits a considerable increase in diameter, and the shaft 17 is moreover fixed to parts which are extremely rigid in themselves. The second bearing 18 is simply designed to withstand moments around the centre of gravity of the rotating part; the lesser rigidity of the journal 17a of the fixed shaft,

i with two coaxial sleeves 35 and 36.

is arranged between the inner periphery of the rotating between the two bearings 1S and 19, is nevertheless sufficient to ensure this latter function. With this arrangement, high natural frequencies have been obtained in spite of the small diameter of the ball bearings.

It is to be observed, moreover, that the rotary part of the pump remains centred without play both under normal running conditions and during stantingup, owing to the absence of any radial expansion opposite the bearmg 19. This result is obtained as a result of the arrangement of the plate 16 in the form of a disc and which is off-set axially with respect to the bearing 19. Thus, the centrifugal force acting in the plane of the plate 19 scarcely has any further influence at the plane of the hearing 19 as a result of the elastic deformation of the cylindrical portion of the sleeve 13 which separates them.

As regards centring on the bearing 13, it is to be noted that the radial deformations are negligible on the bore corresponding to the outer diameter of that bearing as the cover 15 which supports it is solid.

It is to be noted, finally, that the rotating discs 1 are centered solely by the tie rods 7, the assembly of which forms a hub of large diameter, thus enabling any spacer to be dispensed with and, consequently, any relative sliding of the parts which do not undergo identical deformations to be avoided. Moreover, the plates 9 and 10 can be calculated in such manner that there is no relative sliding between them and the rotating discs 1.

So as not to subject the bores in the discs 1 through which the tie rods 7 extend to too heavy ovalising stresses at high speeds of rotation, a certain balancing of the masses is obtained owing to the portions of extra thickness 11 on the inner periphery of the discs 1 and to the portions of extra thickness 9a and 10a on the outer periphery. of plates 9 and 10. v

The apparatus formed in this way has, at opposite ends, on the one hand an upstream orifice 31 of large conductance connected to the chamber to be evacuated in which.

it is desired to obtain the high vacuum and, on the other hand, a downstream orifice 32 connected to a conventional vacuum backing pump (not shown) designed to create a preliminary vacuum. Thus, the molecules flow in the direction indicated by the arrows, that is to say along a quasi-rectilinear mean path in the direction of the axis of rotation X-Y, from the suction end to the delivery end. In particular, the absence of bends at the suction end, and therefore at the end where the highest vacuum is located, is an appreciable advantage. With such an arrangement, the losses in therate of flow or delivery'in the known devices, which are due to constricted or throttled sections or to non-rectilinear paths, "are eliminated.

The modified construction shown in FIGURE 2 is principally distinguished from the foregoing form of embodiment in that its axis of rotation X-Y is vertical, the

flow of the molecules taking place in the general direction from top to bottom.

Moreover, the stack'of rotating discs 1 is clamped, by means of the tie rods'7, between two end plates 33 and 34. These plates, which are of annular form with peripheral edges 33a and 34a of extra thickness, are integral A slight clearance discs 1 and the outer surface of the sleeves 35 and 36, again with the aim of avoiding local sealed-elf areas in the space B. The latter is moreover placed in communication with the lower space H of the pump through ducts 37 formed through the plate 34.

The sleeve 35 which is closed'by an upper head or end 38 and the sleeve 36, which has no head or end, are connected to one another by a central disc 39 having orifice (not shown) placing the internal spaces I and I of the two sleeves in communication. These spaces are isolated from the space B outside the sleeves and which is itself isolated from the top space F under high vacuum.

In the central disc 39 there is housed the end of the .driving shaft 40 of the pump. The shaft 40 rotates in roller bearings 41 and 42 carried by a cylindrical drum 43 integral with a frusto-conical base 44 fixed rigidly to the casing 5 of the apparatus by means of a collar 45. The drum 43 is interposed, with the required clearances, between the shaft 40and the sleeve 36 and has its upper end set in with respect to the central disc 39 for attachment of the rotor of the pump to said driving shaft 40. The effect of this arrangement is to cause the bearings to operate with their rotating races on the inside and their fixed races on the outside, which afiords an appreciable advantage in view of the very high speeds of rotation and the severe mechanical conditions of which the pump is axial thrust due to the weight of the rotary assemblybeing transferred to the lower bearing 42 which may have an oblique contact bearing contact. The security of the assembly in the axial direction is ensured by a stop 46 which comes into play only in quite exceptional circumstances if the force directed upwardly and originating from the differential pressures should become preponderant over the weight of the rotor or if the bearing 42 should fail.

The pump is again driven by a built-in electric motor of any type, for example of the axial plane gap type and with a fiat winding having lamellar conductors; this winding is formed on discs 47 which form part of the stator of the motor and are fixed to the base 44 of the drum, while the rotor 48, constituted, for example, by a torus of magnetic ferrite, is fastened to a shaft 49 rotating in bearings (not shown) carried by the stator of the motor.

The shaft 49 of the motor is coupled to the shaft 40 of the vacuum pump through a splined joint 50 adapted to transmit a pure torque and permitting a certain freedom of relative movement of the rotors of the two machines. Moreover, this connection offers the possibility of removing one of the machines without affecting the other. In short, the vacuum pump and the electric motor constitute two machines independent of one another, apart from the coupling such that the motor transmits a pure torque to the pump.

At the end of the shaft 49 of the electric motor there may be provided a small auxiliary lubricating pump 51, for example of the centrifugal type, exerting suction through an axial duct 52, in a cup or reservoir 53 fixed below the base 44, and delivering into a pipe 54 arranged to permit the wetting or spraying of the bearings. The return of the lubricant to the cup or reservoir 53 takes place under gravity through the inner space K which is maintained at the pressure of the preliminary vacuum by means of a breather pipe 55; any lubricant which has managed to infiltrate into the space I is collected at the bottom of the latter and re-introduced into the inner space K through one or more suitable bores 58.

The oil reservoir is cooled by circulation of water through a coil or a jacket 56 covering the frustoconical base 44. This therefore performs the function of a cold wall in relation to any oil vapour likely to arise and to infiltrate as far as the internal space H and prevents the oil vapour reaching a considerable pressure level. The conduits (not shown) for the inlet and outlet of cooling water extend through said space H and perform the function of a second cold-producing stage superimposed on the preceding one.

It is to be noticed that, as in the arrangement of FIG- URE l, as a result of the sealing between the internal spaces I, J, K which are likely to contain particles of oil and the intermediate space B and of the sealing between the latter and the upstream space F, the bearings 41, 42

6 and the lubricating system are completely isolated from the high vacuum.

Since the c ooling of the electric motor cannot be effected by convection .by reason of its position in the chamber K under vacuum, it is advisable to provide for cooling of the stator 47 by circulation of water; the rotor 48, constituted by a magnetic ferrite, does not require cooling.

The apparatus which has just been described lends itself remarkably to the usual baking or out-gassing operation which consists in heating it so as to promote the emission of the gaseous particles occluded in its walls, without running the latent risk of producing a vaporisation of oil such that the oil vapour may ascend as far as the upstream space F and the chamber to be evacuated.

This reliability is obtained by virtue of the general arrangement of the pump, particularly the following features:

(a) Admission at the top and circulation of the gaseous molecules at the periphery of the pump, with oil reservoir at the bottom and lubricating zone in the centre of the pump;

([1) Complete sealing between the spaces through which particles of oil are likely to pass and the region of high vacuum; and l (0) Cold wall preventing any possible oil vapour from reaching a considerable pressure level.

In order to carry out the out-gassing operation, it is possible, according to the invention, to utilise a heating resistance 57 disposed around the casing 5 and covered with an asbestos jacket (not shown), a preferential heating being advantageously applied to the suction flange 31a and to the upstream stages of the pump where the elimination of the occluded gaseous molecules assumes a critical character. The heat is transmitted by conduction to the fixed outer .parts of the pump and by radiationto the rotor. The temperature levelreached in'the lower central portion of the pump is relatively low and the presence of the cold screen 56 strongly reduces it further.

In the course of the out-gassing process, the assembly is at the pressure of the preliminary vacuum through the action of the conventional vacuum backing pump (not shown) permanently connected to the outlet orifice 32, the evacuation of the molecules emitted by the walls being ensured by operating the turbomolecular pump at low speed, for example at half speed, so as not to subject any parts in an overheated state to excessive mechanical stresses.

Once out-gassing has been completed, the current supplying the heating resistance 57 is cut off, the pump is allowed to cool and, finally, the electric motor 47, 48 is started at full speed until the desired high vacuum is obtained.

As in the case of the embodiment of FIGURE 1, the modified embodiment shown in FIGURE 2 enables extremely high speeds of rotation to be attained by reason of the great bending strength obtained by arranging the bearing 41 close to the centre of gravity of the rotating part, the second bearing 42 being intended merely to withstand moments around the centre of gravity of the rotating part: the rigidity of the portion of the drum 43 between the two bearings is suflicient to ensure this latter function.

What is claimed is:

1. A rotary high-vacuum pump of the molecular type comprising a stationary part, a rotor, means for mechanically transmitting to said stationary part stresses originating from said rotor, a series of discs alternately secured to said stationary part and to said rotor and having apertures for the passage therethrough of gas molecules, a high-vacuum end section with a large-conductance suction orifice located on one side of said series of discs, a preliminary-vacuum end section with a discharge orifice located on the other side of said series of discs whereby the molecules of rarefied gas have from said suction 7 orifice and through said apertures ,a mean path substantially 'without any bend'andpa'rallel to the rotation axis of the pump, and rotor support means excluding any lubricated bearing inf'said high-vacuum section.

'2.' A pump accordingto claim 1,"wherein the mechanical transmission't'o the stationary part of the pump of the stresses originating from the rotor is effected entirely on one and 'the'same side of the pump, namely the preliminary-vacuum end thereof, the rotor being overhung in the direction-of the high-vacuum end.

3. A pump according to claim 1, wherein the mechanical transmission means comprise a main bearing carrying the rotor, positioned close to'its center of gravity and having great bending resistance to radial stresses.

4. A pump'according to claim 3, comprising further a secondary bearing, associated with the main bearing,

tion parallel to the axis of rotation.

6. A pump according to claim 5, wherein the upstream one ofsaid plates is located in front of the first rotating disc and is constituted by, a solid circular part.

7. A pump according to claim 5, wherein the downstream plate occupies an intermediate position in the series of rotating discs.

8. A pump according to claim 5, wherein the plates have extra thickness portions located at the outer periphcries of theplates.

9. A pump according to claim 5, wherein the rotor comprises a plurality of tie rods extending through the rotating discs and plates, distributed regularly around the axis ofrotation of the pump and extending parallel to said axis, said tie rods extending through annular portions of the rotating discs of increased thickness and having plane parallel faces, the rotating discs being firmly clamped against one another at said plane parallel faces.

10. A pump according to claim 9, wherein an extra thickness. inner annular portion is formed on a smaller radiator 'ther'otating discs, the thicknessbf this latter inner portion being very slightly less than that of the first-mentioned'annular portion, so as'tdprovide a small clearance between 'successive inner annular portions.

' '11. A pump according to claim 3, wherein the rotor comprises a disc-supporting platewhich is located in' a -p'laiie' off-set axially with respectto that of the main hear- 'ing, and a sleeve integral with said plate andconnec'ted with said main bearing. s V

12.1A pump according to claim 11, wherein the stator comprises a fixed support carrying the bearing and surrounded by the sleeve, and the pump rotor is mounted in an overhung manner on said fixed bearing support.

13. A pump according "to claim 12, wherein the fixed s'upport,'which is hollow and of relatively large diameter, has an end journal of smaller diameter on which the roto'rbear'ing is mounted.

14. A pump according to claim 12,'wherein the'sle'eve is closed by a solid circular part fixed 'to the sleeveaiid, arranged in front of the adjacent end of thefixed sup- 1 port. 7

15. A pump as claimed in claim 1, comprising'rotor bearing means in a central portion of the pump, means for circulating lubricant within said central portion to lubricate said rotor bearing means, and meansfor fiuidtightly separating said central portion wherein lubricant particles circulate from the high-vacuum-sectionand "at least the peripheral portion ofthe pump adjacent saidsection.'

References Cited hy-the Examiner UNITED STATES PATENTS 2,918,208 12/59 Becker 230 1l8 FOREIGN PATENTS 866,706 10/ 41 France.

LAURENCE V. EFNER, Primary Examiner. ROBERT M. WALKER, Examiner. 

1. A ROTARY HIGH-VACUUM PUMP OF THE MOLECULAR TYPE COMPRISING A STATIONARY PART, A ROTOR, MEANS FOR MECHANICALLY TRANSMITTING TO SAID STATIONARY PART STRESSES ORIGINATING FROM SAID ROTOR, A SERIES OF DISCS ALTERNATELY SECURED TO SAID STATIONARY PART AND TO SAID ROTOR AND HAVING APERTURES FOR THE PASSAGE THERETHROUGH OF GAS MOLECULES, A HIGH-VACUUM END SECTION WITH A LARGE-CONDUCTANCE SUCTION ORIFICE LOCATED ON ONE SIDE OF SAID SERIES OF DISCS, A PRELIMINARY-VACUUM END SECTION WITH A DISCHARGE ORIFICE LOCATED ON THE OTHER SIDE OF SAID SERIES OF DISCS WHEREBY THE MOLECULES OF RAREFIED GAS HAVE FROM SAID SUCTION ORIFICE AND THROUGH SAID APERTURES A MEAN PATH SUBSTANTIALLY WITHIN ANY BEND AND PARALLEL TO THE ROTATION AXIS OF THE PUMP, AND ROTOR SUPPORT MEANS EXCLUDING ANY LUBRICATED BEARING IN SAID HIGH-VACUUM SECTION. 